The present invention relates to a method and apparatus for exposing an image recording medium, such as a thermal printing plate.
FIG. 1 is a side view of a conventional single beam internal drum imagesetter. A laser 1 generates a laser beam 2 which is directed onto an angled reflective surface 3 of a spinning mirror 4. The spinning mirror 4 is rotated by a motor 5 which is mounted on a carriage (not shown). The carriage (not shown) is driven parallel to the axis of a drum 7 by rotation of a lead screw 6. Items 3-6 are housed inside the drum 7. One or more image recording plates (not shown) are mounted on the inner surface of the drum 7. To expose the image recording plates on the drum 7, the motor 5 moves along the axis of the drum 7, and rotates the spinning mirror 4 about the axis of the drum 7 whereby the reflected laser beam 8 exposes a series of circumferential scan lines.
As can be seen in FIG. 2, which is an end view of the apparatus shown in FIG. 1, during the lower 80xc2x0 of its revolution, the reflected laser beam 8 is blocked by the carriage 136. This creates a shadow area 9 which prevents the scanner from exposing a full 360xc2x0 of the drum 7 and reduces the speed and efficiency of the system. The angle of the area outside the shadow area 9 is conventionally known as the xe2x80x9cdrum anglexe2x80x9d.
A known way of improving on the efficiency and scanning time of the system of FIG. 1 is to add a second spinner and a second laser as illustrated in FIG. 3.
FIG. 3 illustrates the lower half 10 of a cylindrical drum. A first mirror 11 and a second mirror 12 are mounted at 180xc2x0 to each other on a common shaft 13 which is rotated by a motor (not shown). A first laser 14 is directed at the spinning mirror 11, and a second laser 15 is directed at the spinning mirror 12. The distance between the reflective surfaces of the spinning mirrors 11,12 is equal to half the length of the drum. The laser 14 directs image radiation to the mirror 11 during one half cycle to expose a line on the upper half of the drum. The laser 15t directs image radiation to the mirror 12 during the next half cycle to expose another line on the upper half of the drum. The process continues until the right-hand spinner 12 has exposed the right-hand upper quarter of the drum, and the left-hand spinner 11 has exposed the left-hand upper quarter of the drum. Therefore the entire upper half of the drum can be exposed in half the time when compared with the system of FIG. 1. In addition the overall efficiency is increased since the lower half of the drum (which includes the shadow area 9) is not exposed.
A problem associated with the system of FIG. 3 is that two lasers 14,15 are required. The cost of lasers can be very high.
In accordance with a first aspect of the present invention there is provided apparatus for exposing an image recording medium, the apparatus comprising a radiation source; a switch comprising an input arranged to receive radiation from the radiation source, and a plurality of imaging outputs, wherein the switch selectively routes the radiation received at the input to a selected one of the imaging outputs; and means for directing the radiation from each imaging output onto the image recording medium to expose the image recording medium.
In accordance with a second aspect of the present invention, there is provided a method of exposing an image recording medium, the method comprising generating radiation in a radiation source; inputting the radiation to a switch having a plurality of imaging outputs; routing the radiation during a first period to one or more selected ones of the imaging outputs; routing the radiation during a second period to one or more different selected ones of the imaging outputs; and exposing the image recording medium with radiation, from the or each selected imaging output.
The present invention provides a routing device which enables a single radiation source to be used in a scanner of the type illustrated in FIG. 3. This results in a much simplified system with reduced cost.
The radiation which exposes the image recording medium is generally encoded with image information to expose a desired pattern of pixels. The radiation may be encoded downstream of the routing device, for instance with an acousto-optic modulator. Preferably however the radiation which is input to the routing device is already encoded, for instance by suitable control of the radiation source. Typically the radiation source inputs radiation in the form of a series of pulses to the routing device. This enables pixels to be exposed on the image recording medium with short, high power pulses, resulting in low thermal leakage.
In a preferred embodiment the radiation source comprises an optical amplifier having a pump energy source. The average power of the optical amplifier can then be conveniently adjusted by adjusting the power input by the pump energy source. The pump energy source may input electrical pump energy into the amplifier, but preferably the pump energy source comprises a radiation source such as an array of laser diodes.
The radiation source may be operated in a continuous wave mode as illustrated schematically in FIG. 4. A power source (not shown) provides a power signal on input line 16. When switch 17 is closed the laser cavity 18 outputs a laser beam 19. A problem with continuous wave mode is that the laser beam 19 cannot have a power any greater than the power on input line 16. This is a particular problem in thermal printing imagesetters where high laser power may be required.
Therefore preferably the radiation source is operated in pulsed mode, as illustrated schematically in FIG. 5. In this case a power source provides a power signal on input line 20 which is input continuously to the laser cavity 21. The laser cavity 21 stores the energy from input line 20 until switch 22 is closed to release the energy in the form of a high power pulsed laser beam 23. As a result, the power of the pulsed laser beam 23 can be higher than the power on input line 20. This enables pixels to be exposed on the image recording medium with short, high power pulses, resulting in low thermal leakage.
An example of a suitable radiation source is shown in FIG. 6. FIG. 6 illustrates a fibre amplifier of the type described in WO95/10868. The fibre amplifier comprises a fibre 30 having a Erbium-Ytterbium doped single-mode inner core 31 and a multi-mode concentric outer core 32. A single mode seed laser 33 directs an encoded laser beam 34 into the inner core 31. Pump radiation is provided by a pump source 35 (an array of multi-mode laser diodes) which is coupled, transversely with respect to the optical axis of the fibre 30, to the outer core 32. The method of coupling the pump source 35 to the fibre 30 is described in detail in WO96/20519. Pump radiation from the pump source 35 propagates through the outer core 32 and couples to the amplifying inner core 31, and pumps the active material in the inner core 31. Thus the fibre optic amplifier provides a highly amplified encoded output beam 36 at the wavelength of the encoded laser beam 34.
The fibre optic amplifier illustrated in FIG. 6 is primarily designed for use in telecommunications in which the encoded input laser beam 34 will not be off for a significant length of time. If the seed laser 33 is off for an extended period, the fibre 30 continues to accumulate energy from the pump source 35, and as a result the fibre 30 will go into spontaneous emission. This problem is common to all pulsed laser sources and as a result pulsed laser sources are generally not used in imaging applications where the laser may be off for an extended period of time.
In order to solve this problem, the apparatus preferably further comprising an energy dump; and means for directing the radiation from the radiation source either to the energy dump or to the image recording medium. This solves the spontaneous emission problem by providing an energy dump which is utilised to prevent excessive build up of energy in the radiation source.
The means for directing the radiation to the energy dump or the image recording means may comprise a switch. However it may be difficult for a conventional switch to operate at the switching frequency required. Therefore preferably the radiation source comprises a data radiation source and a dump radiation source which generate encoded radiation at respective different wavelengths, and an optical amplifier which amplifies the encoded radiation; and wherein the means for directing the radiation either to the energy dump or to the image recording medium comprises a filter which directs the amplified radiation to the image recording medium or to the energy dump in accordance with the wavelength of the amplified radiation. In this case the apparatus typically further comprises means for encoding the radiation from the dump radiation source whereby radiation is only generated by the dump radiation source when radiation is not being generated by the data radiation source. This increases efficiency and further reduces the risk of spontaneous emission.
The switch typically comprises an electro-optic switch, such as an integrated optic switch. Any suitable radiation source may be used, such as a continuous wave laser or a pulsed laser (for instance the laser of FIG. 6).
The radiation may be transmitted through air to the image recording medium, but preferably the means for directing the radiation from each imaging output onto the image recording medium comprises a plurality of fibre-optic cables, each coupled to a respective one of the imaging outputs. This arrangement improves coupling efficiency, reduces alignment problems, and makes the apparatus safer by confining the imaging radiation beams (which may have dangerously high power). Preferably the radiation source comprises a fibre laser which provides an output suitable for coupling to the fibre-optic cables.
The apparatus may be used in a conventional imagesetter. However it is particularly suited to a thermal imagesetter in which the radiation source generates radiation of a wavelength and power suitable for exposure of a thermal imaging plate. Suitable wavelengths are in the infra-red region. Typically the image recording medium has a media sensitivity of 50-200 mJcmxe2x88x922. Typically the average power delivered by the radiation source at the image recording medium is 2-10W (in the case where the image recording medium is exposed uniformly).